For electronic devices, a method in which they are manufactured by connecting a module board mounted with various kinds of electronic parts to a motherboard, etc., is widely known. Such a module board is typically connected to a motherboard by solder-joining a solder-joint conductor region placed on a wiring conductor on its surface and an electrode terminal of the motherboard, thereby allowing the electronic parts mounted on the module board to function.
Although copper and copper-based alloys, which have low electric resistance, are the main current of the wiring conductor of module boards, copper oxidizes in the air, decreasing solder connection property. As surface treatment for preventing the oxidization of wiring conductors, therefore, electroless gold plating after performing electroless nickel plating has been performed. In this method, however, it is known that the surface of nickel may be corroded along with the precipitation of gold, thereby decreasing solder connection property. A method for forming an electroless palladium plating film as a protective layer between an electroless nickel plating film and an electroless gold plating film (for example, Patent Document 1) and a method for forming an imidazole compound film directly on a wiring conductor (for example, Patent Document 2) have been performed.    Patent Document 1: Japanese Patent Application Laid-Open No. H09-8438.    Patent Document 2: Japanese Patent Application Laid-Open No. H03-28381.